Structured Body for an Anode Used in Fuel Cells

ABSTRACT

The structured body intended for use for an anode ( 1 ) in fuel cells, includes a structure formed by macro-pores and an electrode material. The macro-pores form communicating spaces which are produced by using pore forming materials. The electrode material includes skeleton-like or net-like connected structures of particles which are connected by sintering and which form two reticular systems which interengage: a first reticular system made of ceramic material and a second reticular system which contains metals to effect an electrical conductivity. The electrode material has the properties so that, with a multiple change between oxidizing and reducing conditions, substantially no major property changes occur in the ceramic reticular system, and an oxidization or reduction of the metals occurs in the second reticular system.

BACKGROUND OF THE INVENTION

The invention relates to a structured body for an anode used in fuelcells which is briefly termed an anode structure in the following. Theinvention also relates to a high temperature fuel cell having such ananode structure and to a method of manufacturing the anode structure inaccordance with the invention.

A high temperature fuel cell (SOFC fuel cell) is known from DE-A-19 819453 in which an anode substrate forms a support structure. Anintermediate anode layer, a solid electrolyte, preferably in the form ofa very thin layer, and a layer-like cathode are applied to this supportstructure. The anode substrate and the intermediate anode layer aremanufactured from the same material, namely from a porous cermet whichconsists of the ceramic material YSZ (yttrium stabilized zirconiumoxide), which is also used as the solid electrolyte, and nickel.Electro-chemical reactions take place at so-called three-phase points(nickel/electrolyte/pore) in the contact region between the anode andthe electrolyte in which nickel atoms are oxidized by oxygen ions (O²⁻)of the electrolyte and these are reduced again by a gaseous fuel (H₂,CO), with H₂O and CO₂ being formed and the electrons released in theoxidization being passed on by the anode substrate. To obtain a largedensity of points for these three-phase points, a composition of theintermediate anode layer is provided for which the ratio of theproportions by volume of nickel, YSZ and pores lies close to 1:1:1.

The aim of the largest possible density of the three-phase points is,however, of lesser importance with respect to a further problem, namelywith respect to the requirement that the anode should have a so-calledredox stability. The redox stability relates to the properties of theelectrode material with respect to a multiple change between oxidizingand reducing conditions. On the one hand, this change, which is brieflytermed a redox change in the following, should not result in any majorchanges in properties for the ceramic components. On the other hand, anirreversible change, i.e. an ageing of the metallic components as aresult of the redox change, should be influenced by means of theconstant ceramic components such that the electrical conductivity of theelectrode material is largely maintained. With such an ageing, a graingrowth of the nickel takes place in which large crystallites grow at thecost of small ones and so allow gaps to arise in electrically conductiveconnections of the anode structure.

The redox stability is very important in practice because, according toexperience, it is not possible to keep a battery with fuel cells incontinuous operation. At each operation stop, the supply of the fuelmust be stopped for safety reasons. When the gaseous fuel is absent,oxygen penetrates onto the anode sides of the fuel cells and thepreviously reduced state of the nickel changes into the oxidized state.When fuel cells are used in domestic technology for the purpose of thesimultaneous production of electrical and thermal energy, around 20interruptions to operation can be expected per year. A fuel cell must beusable for around five years for economical reasons. The fuel cell mustthus only age so fast that up to 100 redox changes are possible.

However, in addition to the redox stability, good gas permeability ofthe anode structure is also important, as is—with respect to commercialuse—a production of the anode structure which should be economical.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an anode structure which issufficiently good with respect to redox stability, gas permeability andefficiency for use in fuel cells. This object is satisfied by the anodestructure in accordance with the embodiments of the present invention.

The structured body, which is provided for use for an anode in fuelcells, has a structure formed by macro-pores and an electrode material.The macro-pores form communicating spaces which are produced by poreforming materials. The electrode material includes skeleton-like ornet-like connected structures of particles connected by sintering whichform two reticular systems which interengage or cross into one another:a first reticular system of ceramic material and a second reticularsystem which includes metals to bring about electrical conductivity. Theelectrode material has the properties so that, with multiple changesbetween oxidizing and reducing conditions, no major changes inproperties occur in the ceramic reticular system, on the one hand, andan oxidization or reduction of the metals results in the secondreticular system, on the other hand. In addition, the two reticularsystems together form a compact structure which includes micro-pores inthe oxidized state whose proportion by volume is less than 20%,preferably less than 5%, with respect to the electrode material.

The term “reticular system” has been introduced here. This is to beunderstood as a skeleton-like or net-like connected structure ofparticles. Due to the connection of the reticular system, this is givena structural stability and/or electrical conductivity. No chemicalchanges can take place in the reticular system so that structuralstability is present. This is the case for the ceramic reticular system.The second reticular system has a structure which changes due to theredox change such that only a low structural stability is present. Thefunction of the second reticular system as an electrically conductiveconnection is maintained due to the structural stability of the firstreticular system. The two reticular systems result in a natural mannerin the form of a statistical distribution of constituting particles ifthese particles are prepared such that they each have a narrow sizespectrum for the two particle kinds when the proportion by volume foreach reticular system amounts to at least 30% and when the particles aremixed homogeneously with one another. (However, even relatively largeparticles, which are included in insular manner in a matrix of fineparticles, may be mixed in the ceramic reticular systems.) The systemformed by the pores is likewise a reticular system. This reticularsystem results in the required gas permeability of the anode structure.

In a preferred embodiment, the anode structure in accordance with theinvention has both the function of a support structure and the functionof the above-mentioned intermediate anode layer. It can, however, alsoonly form the supporting part of the anode which serves as a supportstructure for an intermediate anode layer. The body in accordance withthe structure must be made so strong as the support structure that itwithstands a mechanical load of, for example, 20 kPa; a load of thisorder of magnitude is typically present on installation in a stack-likearrangement of fuel cells.

The present invention enables various advantageous anode structures.Furthermore, the present invention enables the formation of a hightemperature fuel cell having these advantageous anode structures.

The invention will be explained in the following with reference to thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a fuel cell;

FIG. 2 is a cross-section through a porous material which can be used asan anode structure in accordance with the invention;

FIGS. 3, 4 are sections from two reticular systems, namely skeleton-likeor net-like connected structures of particles;

FIG. 5 shows the two reticular systems of FIGS. 3 and 4 which areinterengaged; and

FIG. 6 shows an anode structure in whose electrode material largerparticles of ceramic material are incorporated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a high temperature fuel cell such as is shown schematically in FIG.1, electrode reactions are carried out to produce an electric current I;that is, reducing reactions arise on or in an anode 1, wherein water orcarbon dioxide arise from hydrogen and carbon monoxide, which form thegaseous fuel; and oxidizing reactions arise on a cathode 2 in whichionic oxygen O²⁻ is formed—while taking up electrons e⁻ from a metallicconductor 20 and a pole 24—from molecular oxygen of a second gas flow(e.g. air: O₂ and N₂). The oxygen ions move through a solid electrolyte3, which separates the two electrodes 1, 2 in a gas impermeable mannerand which is conductive for the oxygen ions at temperatures above 700°C. The reducing anode reaction takes place with the oxygen ions whileemitting electrons to a further metallic conductor which produces aconnection to a pole 14. A consumer 4 is arranged between the poles 14,24 which load the fuel cell with a resistance R. The voltage U betweenthe poles 14, 24 is produced by a stack of cells connected in series inthe practical application of the fuel cell.

In a preferred embodiment, the anode structure 1 has a heterogeneousdesign made up of a homogeneously structured, porous support structure10 and a more compact marginal zone 11. The pores of the supportstructure 10 are macro-pores 100 and micro-pores 110: see FIG. 2. Themarginal zone 11 in the example shown includes only micro-pores 110. Theadjoining layers, that is, the preferably thin solid electrolyte layer 3and the cathode 2, can be manufactured, for example, by means of athermal injection method; they can also be manufactured while using ascreen printing method. The material for the electrodes 1 and 2 must beusable at an operating temperature of up to 1000° C. The macro-pores 100of the anode structure 1 form communicating spaces which result in apermeability for the gaseous fuel which is adequate with respect to theelectrode reactions. This permeability exists up to the marginal zone 11which forms a boundary zone beneath the electrolyte layer 3. A furthergas permeability is given in this border zone 11 by the micro-pores 110.The porosity given by micro-pores 110 is larger than shown in thereduced state of the anode structure 1.

The anode structure 1 in accordance with the invention has a structureformed by the macro-pores 100 and an electrode material 5. Themacro-pores 100 form communicating spaces which are produced by poreforming materials. The electrode material 5—see FIGS. 3 to 5—includesskeleton-like or net-like connected structures made up of particles 60and 70 connected by sintering. These particles 60, 70 form two reticularsystems 6 and 7 (FIG. 5) which are crossed into one another, namely afirst reticular system 6 (FIG. 3) of ceramic material and a secondreticular system 7 (FIG. 4) which includes metals to effect electricalconductivity. In accordance with the invention, the electrode material 5has the properties that the following applies with a multiple changebetween oxidizing and reducing conditions (redox change): on the onehand, no chemical or structural changes occur in the ceramic reticularsystem. On the other hand, an oxidization or reduction of the metalscontained in the particles 70, where ageing occurs, results in thesecond reticular system due to the redox change. The two reticularsystems 6, 7 together form a compact structure which includesmicro-pores 110 in the oxidized state whose proportion by volume is lessthan 20% with respect to the electrode material 5. This proportion byvolume is preferably less than 10% or, even better, 5%. During thereduction of the metals, the dimensions of the particles 70 reduce inthe second reticular system 7; further micro-pores (not shown) thereforeoccur. On a repeated oxidization of the metals, these furthermicro-pores, which should be designated by “type II micro-pores”,disappear. The micro-pores 110 of the oxidized anode structure 1 are“type I micro-pores”. The “type II micro-pores” have a lower influenceon the structural stability of the electrode material 5 than the “type 1micro-pores”.

As was determined empirically, reticular systems result when theparticles 60, 70 have diameters whose mean value d₅₀ is less than 1 μmprior to sintering (d₅₀=50% by volume of the particles have a diameterlower than d₅₀). In this connection, micro-pores arise whose diametersare smaller than 3 μm. The redox stability becomes even better forsmaller diameters. An anode structure is preferably manufactured suchthat micro-pores arise with diameters lower than 1 μm. The proportion byvolume of the ceramic reticular system—with respect to the electrodematerial—should amount to at least 30%. So that the ageing does not tooquickly result in an intolerable loss in electrical conductivity, theproportion by volume of the second reticular system should be largerthan that of the ceramic reticular system, for example larger by atleast a factor of 1.5.

The particles must be put into a sufficiently fine form for theformation of the reticular systems. This can be done, for example, bygrinding a coarse grain powder. Coarser particles can—if necessary—beremoved from the ground product by classification (e.g. by screening).The required powder quality is obtained, for example, by reprocessingthe powder by means of spray drying. Suitable finely dispersed particlescan also be prepared using methods of nanotechnology: for example bymeans of reaction spray methods, spray flame pyrolysis, depositionmethods or sol/gel methods.

If zirconium oxide, YSZ, is provided as the material for theelectrolyte, then the same ceramic material is advantageously also usedfor the anode structure, i.e. for its first reticular system. Nickeloxide particles, which contain up to 10% by weight of materials otherthan nickel oxide, are preferably used for the second reticular system.Such nickel oxide particles, which are available on the market as dyecomponents, are available as low-cost raw material. These dye particlesadvantageously also contain materials which act as sintering aids in thesintering of the anode structure.

It is of advantage in the manufacture of the anode structure inaccordance with the invention for additives of at least one kind to becontained in the second reticular system. Such additives can serve as asintering aid on sintering. Further additives can impede damaging graingrowth as inhibitors during the operation of the fuel cell. Oxides orsalts of Ni, Mn, Fe, Co and/or Cu can be used as sintering aids and MgOas the inhibitor of grain growth.

The anode structure in accordance with the invention must havecommunicating spaces which enable permeability for the gaseous fuelwhich is adequate with respect to the current supplying electrodereactions. This gas permeability is brought about using macro-poreswhose diameters lie in the region between 3 and 20 μm. The macro-porescan be manufactured using pore forming materials. For this purpose,particles or fibers of an organic material, in particular cellulose, areused. This organic material is decomposed during sintering which iscarried out under oxidizing conditions; the decomposition productsevaporate.

In a suitable method of manufacturing the anode structure in accordancewith the invention, the procedure is as follows: particles of ceramicmaterial (e.g. YSZ) and of a metal oxide (e.g. green or brown nickeloxide particles) are put into a sufficiently fine form for the formationof the reticular systems by grinding and classification. A homogeneousmixture in the form of a slurry is formed from the particles, the poreforming materials and a liquid. The slurry is cast to form a layer. Ifthe slurry is cast in an absorbent mold, some of the liquid is removedfrom it. At the same time, a marginal zone arises in which a lack ofpore forming materials is present. An inhomogeneous structure thusresults such as is shown in FIG. 2.

After a complete removal of the liquid from the slurry layer by drying,sintering is carried out, with the sintering preferably being carriedout together with a solid electrolyte applied to the layer. There is anoptimum sintering temperature at which a structure with smallmicro-pores arises, on the one hand, and an unfavorable influencing ofthe reticular structure remains minimal, on the other hand. The largerthe sintering temperature is, the lower is the density of themicro-pores, but the more strongly the reticular structure is impaired.

FIG. 6 shows an anode structure in whose electrode material 5 largerparticles 60′ of ceramic material are incorporated. The coarse particles60′ are incorporated in an insular manner in a matrix of the fineparticles 60, 70 (see FIG. 5). The manufacturing costs can be reducedwith the coarse particles 60′, whose diameters lie in the range between2 and 10 μm, because less material has to be put into the required fineform. A greater strength of the ceramic reticular system can also beobtained with added coarse particles if a lower proportion by volume ofmicro-pores results due to the coarse particles.

The anode structure in accordance with the invention is made areally,for example in plate-shape or shell-shape. In plate-shape, it has alarge extent in two dimensions; in a third dimension, it has a thicknessof relatively small extent which is, however, larger than 0.5 mm. Therequired supporting force of the anode structure is achieved with thisthickness.

The ceramic reticular system (6) can consist, in addition to thestabilized zirconium oxide, YSZ, also of aluminum oxide Al₂O₃, oftitanium oxide TiO₂, of doped cerium oxide CeO₂, of magnesium oxide MgO,and/or of a spinel compound. Oxides of the substances Y, Ti, Gd and/orCo can be used as doping means for the CeO₂.

With the manufacturing method in accordance with the invention andunlike the high temperature fuel cell described in DE-A 19 819 453, ananode structure corresponding to the anode substrate and theintermediate anode layer can be manufactured in common working steps.This is obviously a simplification of the method which has a positiveeffect on the manufacturing costs.

Further methods known per se can also be used for the manufacture of theanode structure in accordance with the invention: film casting, rollpressing, wet pressing or isostatic pressing.

1-9. (canceled)
 10. A method of manufacturing a structured body for an anode (1) for use in fuel cells, with the structure body comprising a structure formed by macro-pores (100) and an electrode material (5) whose macro-pores form communicating spaces which are produced by means of pore forming materials and whose electrode material includes skeleton-like or net-like connected structures of particles (60, 70) which are connected by sintering and which form two reticular systems (6, 7) which interengage, namely a first reticular system (6) made of ceramic material and a second reticular system (7) which contains metals to bring about an electrical conductivity, and with the electrode material having the properties that, with a multiple change between oxidizing and reducing conditions, substantially no major property changes occur in the ceramic reticular system, on the one hand, and an oxidization or reduction of the metals results in the second reticular system, on the other hand, and the two reticular systems together form a compact structure which contains micro-pores (110) in the oxidized state whose proportion by volume is less than 20%, preferably less than 5%, with respect to the electrode material, wherein the particles (60, 70) are brought into a sufficiently fine form for the formation of the reticular systems (6, 7) by grinding and classification, a homogeneous mixture in the form of a slurry is formed from the particles with pore forming materials and a liquid, the slurry is poured to form a layer and the layer is sintered after removal of the liquid.
 11. A method in accordance with claim 10, wherein the ceramic reticular system (6) can include relatively coarse particles (60′) with diameters between 2 and 10 μm; wherein the further particles (60, 70) have diameters in the unsintered state whose mean value d₅₀ is smaller than 1 μm; wherein the micro-pores (110) have diameters smaller than 3 μm, preferably smaller than 1 μm; and wherein the proportion by volume of the ceramic reticular system (6) with respect to the electrode material amounts to at least 30%.
 12. A method in accordance with claim 10, wherein the ceramic reticular system (6) consists of zirconium oxide stabilized with yttrium, YSZ, of aluminum oxide, Al₂O₃, of titanium oxide, TiO₂, of doped cerium oxide, CeO₂, of magnesium oxide, MgO; and/or of a spinel compound; and wherein particles (60, 70) are used, in particular for the electrode material (5), which consist of stabilized zirconium oxide, YSZ, for the first reticular system or which include at least 90% by weight of nickel oxide for the second reticular system.
 13. A method in accordance with claim 10, wherein the macro-pores (100) have diameters in the range from 3 to 20 μm.
 14. A method in accordance with claim 10, wherein particles or fibers of an organic material, in particular cellulose, are used as pore forming materials for the particles and evaporate on sintering with oxidizing conditions.
 15. A method in accordance with claim 10, wherein additives of at least one kind are included in the second reticular system (7), with the first or only kind serving as sintering aids in sintering and a second kind acting as an inhibitor to grain growth during the operation of the fuel cells.
 16. A method in accordance with claim 15, wherein oxides or salts of Ni, Mn, Fe, Co and/or Cu are used as sintering aids and/or MgO is used as the inhibitor of grain growth.
 17. A method in accordance with claim 15, wherein the structured body is made in plate-like or shell-like form and has a plate thickness or shell thickness which is larger than 0.5 mm.
 18. A method in accordance with claim 10, wherein the electrode material (5) can be used at an operating temperature of up to 1000° C.; wherein the structured body for use as an anode forms a support structure (10, 11) for a thin solid electrolyte layer (3) and a cathode (2); and wherein the communicating spaces of the structured body enable a permeability for a gaseous fuel which is sufficient with respect to current supplying electrode reactions, and indeed up to a boundary zone (11) under the electrolyte layer in which a further gas permeability is given by the micro-pores (110) of the electrode material.
 19. A method in accordance with claim 10, wherein the sintering of the layer formed is carried out together with a solid electrolyte layer applied to the layer formed.
 20. A method including using the structured body manufactured in accordance with claim 10 as an anode of a fuel cell. 